A ball screw in which balls are disposed to be capable of carrying out a rolling motion between the screw shaft and the nut member contributes to reduce a coefficient of friction when the screw shaft rotates with respect to the nut member, in comparison with a screw performing a sliding contact with no ball interposed, so that such ball screw has actually been used for a positioning mechanism or feed mechanism of a machine tool or a steering gear of an automobile.
In the ball screw, a number of balls are interposed between a ball rolling groove, in form of spiral, formed on an outer peripheral surface of the screw shaft and a loaded ball rolling groove, in form of spiral, formed on an inner peripheral surface of the nut member. When the screw shaft is rotated relatively to the nut member, a number of balls roll on the ball rolling groove of the screw shaft and the loaded ball rolling groove of the nut member. The ball rolling to one end of the loaded ball rolling groove of the nut member is scooped up by a return pipe in which a ball return passage is formed so as to connect one end of the loaded ball rolling groove and the other one end thereof and then to return the ball to the original position of the loaded ball rolling groove, thus the balls circulating.
In these days, roller screws utilizing rollers in place of balls have been provided. Although the ball rolls in every direction, the roller has a limit in its rolling direction. Because of this reason, a circulation passage of the roller screw may have a complicated structure.
In a Patent Publication 1, it is disclosed that a roller circulation passage, having a rectangular shape in section, for circulating the roller is formed by connecting one and the other ends of the spiral groove of the nut member, and this roller circulation passage is composed of linear roller scoop-up portions communicated with both ends of a roller rolling portion of the spiral groove of the nut member and a roller circulation portion communicated with these roller scoop-up portions. The roller scoop-up portion is composed of a split circulator member. The roller circulation portion for communicating the roller scoop-up portions is constructed by assembling a return plate, to which a circulation groove, having V-shaped section, is formed, to the outer peripheral surface of the nut member having a circulation groove having V-shaped section. The roller scoop-up portion serves to spirally guide the roller so that the rolling attitude of the roller changes between the spiral groove and the roller circulation portion of the nut member. The roller circulation portion serves to linearly guide the roller from one scoop-up portion to another scoop-up portion without changing its rolling attitude.
Patent Publication 1: Japanese Patent Laid-open Publication No. HEI 11-210858.